The present invention relates to a method for manufacturing cladding tubes of a zirconium-based alloy for fuel rods for nuclear reactors.
As cladding tubes for fuel rods for nuclear reactors there are normally used thin-walled tubes of zirconium-based alloys, known under the name Zircaloy. These alloys contain alloying materials such as tin, iron and nickel. In Zircaloy the .alpha.-phase is stable below 790.degree. C., the .beta.-phase is stable above 950.degree. C., whereas a two-phase region, the .alpha.+.beta.-phase region, occurs between 790.degree. C. and 950.degree. C. In the .alpha.-phase the zirconium atoms are arranged in a hexagonal close packed lattice and in the .beta.-phase in a body centered cubic lattice. During so-called .beta.-quenching of Zircaloy to bring about the desired properties such as improved corrosion properties of the material, the material is heated to a temperature in the .beta.-phase region and rapidly cooled to a temperature in the .alpha.-phase region.
During conventional manufacture of cladding tubes of Zircaloy, a .beta.-quenching of the material is performed after forging of ingots into rods. After the manufacture of extrusion billets of the rods, the billets are extruded in the .alpha.-phase region at a temperature of 680.degree. C., whereafter the extruded product is subjected to cold rollings in a number of steps and, between two consecutive cold rollings, an annealing, intermediate annealing, at 625.degree.-700.degree. C. to enable the subsequent cold rolling. After the last cold rolling step, a final annealing is performed to give the material the desired properties. The final annealing may be performed at temperatures of 400.degree.-700.degree. C.
Tubes manufactured from Zircaloy under the conditions used so far have generally proved to possess sufficient resistance to corrosion under the operating conditions prevailing in a nuclear reactor. However, the development proceeds towards an increasingly higher utilization of the fuel, which means longer operating times for the fuel assemblies. The cladding material will therefore be subjected to the corrosive water for a longer period of time than what has been normal previously, which results in an increased risk of corrosion damage. It has therefore been a desire to achieve better corrosion properties in the alloys used without this involving unfavourable changes of the mechanical properties.
It is previously known, among other things from U.S. Pat. No. 4,238,251, that by .beta.-quenching of a finished tube of Zircaloy it is possible to improve the resistance of the tube to so-called accelerated nodular corrosion in water and steam of high pressure. As will be clear from U.S. Pat. No. 3,865,635, tubes of Zircaloy having good mechanical properties may be achieved by .beta.-quenching of the extruded product before this is subjected to the final cold rolling operation.
The exact reason for the improved resistance to accelerated nodular corrosion achieved by .beta.-quenching has as yet not been completely established. It is considered, however, that the improvement is related to the size and distribution of the intermetallic compounds in the material. The intermetallic compounds, so-called second phases, consist of chemical compounds containing, besides zirconium, primarily the elements iron, chromium and nickel and they exist in the form of particles. The dissolution and precipitation process accomplished by the .beta.-quenching results in a reduction of the size of the particles as well as in a redistribution from evenly distributed particles to particles constituting arrays at the grain boundaries of the .alpha.-grains formed during the .beta.-phase transformation.
A .beta.quenching of the finished cladding tube results in a reduction of the ductility of the tube, which involves a disadvantage with the method. A .beta.-quenching of the extruded product prior to cold rolling into the final dimension gives less deterioration of the mechanical properties of the finished tube. A .beta.-quenching, however, regardless of whether it takes place on a finished tube or prior to the final cold rolling step, results in an impaired yield because of the increased amount of scrap and further because of material losses since a .beta.-quenching leads to the formation of an oxide layer on the surface of the tube, which must be removed.
According to the present invention, it has proved to be possible to manufacture cladding tubes for fuel rods for nuclear reactors having at least as good resistance to nodular corrosion as the best previously known cladding tubes and at the same time a better ductility than such cladding tubes. Compared with previously known methods of manufacturing cladding tubes while using .beta.-quenching after the extrusion, using the present invention, which also includes .beta.-quenching, results in an improved yield because of reduced scrap and further reduced material losses since the oxides formed can be removed on a smaller surface by performing the .beta.-quenching in an earlier stage of the manufacturing process.